1. Technical Field
The present invention relates to a water jet peening method and an apparatus thereof, and more particularly, to a water jet peening method and an apparatus thereof suitable for improving tensile residual stress of a structure in a nuclear reactor to compressive residual stress by water jet peening.
2. Background Art
When there exists residual stress in close to a surface of a welding portion and a heat-affected zone of a structural member composing a nuclear reactor, a process of executing water jet peening (hereinafter, referred to as WJP) for the welding portion and heat-affected zone thereof to improve tensile residual stress existing in the close to the surface of the structural member to compressive residual stress is performed. In a state that the structural member the stress of which is to be improved is immersed in water, the WJP is executed by injecting a high-pressure water jet from a nozzle in the water. A shock wave is generated due to collapse of bubbles included in the injected water jet. The shock wave impacts on the surface of the structural member in the water, thus the tensile residual stress in the close to the surface of the structural member is improved to compressive residual stress. Therefore, the generation of stress corrosion cracking (SCC) in the structural member is suppressed. The stress improvement method by the WJP is described, for example, in Japanese Patent 2841963, Japanese Patent 3530005, Japanese Patent Laid-Open No. 8 (1996)-71919, and Japanese Patent Laid-Open No. 6 (1994)-47668.
In the execution of the WJP for the structural member, a method for confirming execution condition of the WJP is proposed in Japanese Patent Laid-Open No. 8 (1996)-71919 and Japanese Patent Laid-Open No. 6 (1994)-47668.
In Japanese Patent Laid-Open No. 8 (1996)-71919, the WJP is executed for a pipe which is attached to bottom of a reactor pressure vessel and passes through the bottom. The WJP is executed for the place of the pipe existing in the reactor pressure vessel. A high-pressure water jet is injected from the nozzle existing in the water in the close to the WJP execution object of the pipe and the shock wave generated due to collapse of bubbles included in the water jet impacts on the surface of the pipe in the reactor pressure vessel. An AE (acoustic emission) sensor attached to an outer surface of the pipe outside the reactor pressure vessel detects an acoustic signal generated at the time of impact of the shock wave with the place of the pipe in the reactor pressure vessel during execution of the WJP and outputs an AE signal (acoustic power). Whether the residual stress is sufficiently improved for the pipe objected for the WJP or not is confirmed based on the AE signal. When the residual stress is not improved sufficiently, the injection condition (injection s pressure) of the water jet to be injected is controlled and the nozzle position is adjusted.
Japanese Patent Laid-Open No. 6 (1994)-47668 describes that at the time of execution of the WJP, a piezo-electric ceramics (PZT) sensor is attached to a nozzle in the proximity of a jet outlet of the nozzle for injecting a high-pressure and high-speed water jet. When the high-pressure water jet is injected from the nozzle, the PZT sensor detects a shock pulse (a cavitation generation event) generated in the nozzle. The frequency distribution of a signal output from the PZT sensor is analyzed based on the detected shock pulse by a frequency analyzer. A decision apparatus inputting analytical results of frequency distribution outputs a control signal based on comparison results of peak frequency and amplitude thereof obtained from the frequency distribution with their set values. The distance between the nozzle and a surface of a WJP execution object is adjusted based on the excellence peak frequency and the discharge pressure of the pump for supplying water to the nozzle is adjusted based on the peak frequency.